Mata is expected to have his medical on Saturday and, unless there are any hitches, he should make his United debut in Tuesday's Premier League clash against Cardiff at Old Trafford.
United manager David Moyes desperately needed a high-profile signing to lift the gloom after a miserable start to his reign as Alex Ferguson's successor.
Since replacing Ferguson in July, Moyes has seen United fall 14 points behind Premier League leaders Arsenal, while they also crashed out of the FA Cup in the third round against Swansea and suffered an embarrassing League Cup semi-final loss to Sunderland in midweek.
Moyes had been keen to land a quality midfielder since arriving from Everton and he made failed bids for several top stars, including Barcelona's Cesc Fabregas, before the last transfer window shut in September.
His swoop for Mata should add much-needed creativity and intelligence to United's midfield, although the champions also need a more aggressive ball-winner in the mold of former Old Trafford great Roy Keane.
Mata, who cost Chelsea STG23.5 million ($A45 million) from Valencia in 2011, became available after falling out of favour with Chelsea manager Jose Mourinho.
He won Chelsea's player of the year award for the last two season and has 32 caps for Spain, but failed to impress Mourinho - repeatedly being left on the bench.
Mourinho has given Eden Hazard, Oscar and Willian the main attacking roles and the Portuguese coach conceded Chelsea's system does not suit Mata, who reportedly asked to leave after being substituted during a match at Southampton earlier this month.
It had seemed the deal might be delayed while Chelsea tied up their move for Basel's Egypt winger Mohamed Salah, but Mata has been given the green light to quit Stamford Bridge.
"It's what he wants. It's what he asked us to accept,'' Mourinho earlier on Friday before United confirmed the deal.
"The offer, compared with what the club paid two years ago (when Mata signed from Valencia), was fantastic.
"If Manchester United pay their record for a player it's because they know exactly what they want. They are convinced he's going to be important for them.''
Keen to preserve his place in Spain's World Cup squad, Mata has jumped at the opportunity to help revive United's fortunes.
His signing also underlines the determination of United's owners, the Glazer family, to back Moyes after growing criticism of the Scot's torrid start at Old Trafford.
-AAP
